The broad goal of the proposed studies is to contribute to a growing understanding of the precursors of mathematical knowledge in the first year of life. What is the cognitive foundation upon which complex culturally dependent and symbolic mathematics is built? Research with human adults, children, human infants and non- human animals suggests that there is a phylogenetically and developmentally primitive non-verbal system for representing number. The goal of this competitive renewal is to test the hypothesis that nonverbal numerosity representations serve as a foundation for later developing number skills. To this end we investigate variability in normative development in numerical cognition using behavioral and neurobiological methods and longitudinal designs to assess whether numerical sensitivity in infancy is predictive of variability in numerical cognition in early childhood. Aim 1 examines individual variation in numerical discrimination and the relationship between numerical discrimination and numerical rule learning using new paradigms never before applied to the study of numerical cognition in infancy. Aim 1 also studies infants longitudinally from 6-18 months to assess stability in numerical acuity over development. Aim 2 uses a longitudinal design to explore whether measures of numerical cognition in infancy predict later numerosity discrimination and numerosity-symbol mapping in early childhood. Aim 3 explores the relationship between brain and behavioral measures of number representation in infancy and early childhood and the specificity of neural markers to numerical discrimination in infancy. Beyond studying normal development of numerical acuity in infancy and the relationship between numerical abilities visible in the first year of life and later developing numerical abilities, this research sets the stage for studies of abnormal development by developing early assays that could identify children at risk for developmental dyscalculia (DD). DD affects approximately 6% of the global population (e.g., Badian, 1983;Gross-Tsur, Manor, &Shalev, 1996). A positive correlation between numerical acuity in infancy and early childhood or between numerical acuity in infancy and acquisition of the verbal counting system in childhood could lead to earlier identification of children with DD. PUBLIC HEALTH RELEVANCE: The broad goal of the proposal is to understand the development of non-verbal numerical thinking in infancy using behavioral and neurobiological assays. The studies are designed to assess individual variation and links between numerosity discrimination in the first year of life and later numerical abilities in an effort to set the stage for new assays to identify children at risk for developmental dyscalculia, a disorder that affects approximately 6% of the general public.